Spark plug including an insulator with a front end portion having first and second sections

Abstract

Disclosed is a spark plug including: an insulator having an axial hole formed along an axis of the spark plug; a center electrode disposed in a front end side of the axial hole; and a metal shell fixed around an outer circumference of the insulator, with a front end portion of the insulator protruding frontward from a front end of the metal shell. The front end portion of the insulator consists only of a first section and a second section located adjacent to and frontward of the first section and having an inner diameter larger than that of the first section. The second section has a chamfered area formed on an inner circumferential surface thereof so as to continue to a front end of the insulator.

Having described the invention, the following is claimed: 
     
       1. A spark plug, comprising:
 an insulator having an axial hole formed in a direction of an axis of the spark plug; 
 a center electrode disposed in the axial hole and having a part thereof corresponding in position to at least a front end of the insulator; and 
 a metal shell fixed around an outer circumference of the insulator, with a front end portion of the insulator protruding frontward from a front end of the metal shell, 
 wherein the front end portion of the insulator consists only of a first section located on a rear side thereof and a second section located adjacent to and frontward of the first section and having an inner diameter larger than that of the first section, and 
 wherein the second section has, formed on an inner circumferential surface thereof; a chamfered area continuing to the front end of the insulator. 
 
     
     
       2. The spark plug according to  claim 1 ,
 wherein an inner circumferential surface of the first section includes a connection surface region facing frontward and connected to the second section, and 
 wherein, assuming that, in a cross section of the spark plug taken including the axis, a straight line passes through both ends of a line segment corresponding to the connection surface region, an angle formed between the axis and the straight line on a side frontward of the connection surface region is 75 degrees or greater. 
 
     
     
       3. The spark plug according to  claim 1 ,
 wherein a distance between inner circumferential surfaces of minimum inner diameter parts of the first and second sections in a direction perpendicular to the axis is greater than or equal to 5 μm and smaller than or equal to 500 μm. 
 
     
     
       4. The spark plug according to  claim 3 ,
 wherein the distance between the inner circumferential surfaces of the minimum inner diameter parts of the first and second sections in the direction perpendicular to the axis is greater than or equal to 15 μm and smaller than or equal to 100 μm. 
 
     
     
       5. The spark plug according to  claim 1 ,
 wherein a distance from the front end of the insulator to a rear end of the second section in the direction of the axis is 0.1 mm or greater. 
 
     
     
       6. The spark plug according to  claim 1 ,
 wherein an inner circumferential surface of the front end portion of the insulator has a surface roughness of 1 μm or smaller. 
 
     
     
       7. The spark plug according to  claim 1 ,
 wherein the chamfered area is a C-chamfered area or a R-chamfered area.